A survey has found that British garden ponds are being unwittingly polluted by tap water which is added in order to keep water levels high. Photograph: Ashley Cooper/Corbis

British garden ponds are unwittingly being polluted by people topping them up with tap water, a survey has found.

Around half of 250 ponds examined are in "poorer" condition, three in 10 are "good" and only one in 10 was rated as "excellent", said the organisers of the Big Pond Dip, Pond Conservation.

Water boatmen, beetles, snails, alderflies and damselfly larvae are among the pond life affected by the problem, which occurred in more than half the garden ponds surveyed.

Tap water has much higher levels of nitrates than the level found in natural ponds. While not directly poisoning invertebrates and other wildlife, they can adversely affect their habitat. Nitrates cause excessive nutrients in the water, encouraging plants such as duck weed and blanket weed, which in turn make life a struggle for the submerged plants that are essential for healthy and diverse pond-life.

"This is mainly an issue with low-lying farming landscapes and is therefore widespread, affecting anywhere in the south of England, all of East Anglia and low-lying areas of Yorkshire," said Jeremy Biggs, policy and research director at Pond Conservation. "However, for people who get their tap water from Welsh hills, Scottish mountains or up on the moors, it's not a problem."

In addition to farms, nitrates enter pond water from treated sewage effluent and run-off from urban areas. While the levels of nitrates in drinking water are brought down to safe drinking levels for people, Biggs said, they can often be 10 times the level found in natural ponds.

Pond owners need to be better educated on how to create a healthy environment for species, Biggs added. "People worry about water levels going up and down, but freshwater creatures are perfectly adapted to ups and downs, and those ups and downs make good habitats." Instead of using tap water, he said, people worried about a pond's water level should top them up with rainwater.

Depth of ponds was another wildlife concern raised by the survey. The majority were deeper than 30cm (about 12in), which hinders a variety of species because many garden ponds are small in surface area and result in steeply shelving sides, the opposite of the gently sloping sides most pond-life needs.

"If you're interested in biodiversity, ponds should be no more than a foot deep. Most creatures – such as amphibians, tadpoles and newt larvae – live in just 2cm of water," said Biggs. He added it was a myth that ponds need to be deep to prevent them freezing in winter.

While half of ponds were classified as "poorer", the lowest possible rating in the survey for quality of wildlife habitat, Pond Conservation stressed even those "still provided a useful habitat". Over half of all ponds were visited by dragonflies and damselflies, three quarters had water snails, water beetles and pond skaters, and almost all were visited by amphibians, with common frogs accounting for most records.

The results of the survey, which was undertaken across the country between May and September 2009, comes ahead of a new mass participation water survey starting next week. The 'OPAL water survey' organised by Pond Conservation, the Natural History Museum and University College London, hopes to use public records to produce a better picture of how polluted British ponds, lakes and rivers are. A Environment Department-funded survey earlier this year said eight out of 10 public ponds in Britain are in a "terrible state".

"There are few careful observations of ponds," said Biggs. "Though they are the most familiar freshwater environments to us in one way, they are also the least well-known."